Touch sensing systems (“touch systems”) are in widespread use in a variety of applications. Typically, the touch systems are actuated by a touch object such as a finger or stylus, either in direct contact, or through proximity (i.e. without contact), with a touch surface. Touch systems are for example used as touch pads of laptop computers, in control panels, and as overlays to displays on e.g. hand held devices, such as mobile telephones. A touch panel that is overlaid on or integrated in a display is also denoted a “touch screen”. Many other applications are known in the art. To an increasing extent, touch systems are designed to be able to detect two or more touches simultaneously, this capability often being referred to as “multi-touch” in the art.
A number of touch sensing technologies exists, e.g. resistive, surface acoustic wave, capacitive and optical touch sensing technology. WO2011/028169 discloses an example of an optical touch system capable of detecting a plurality of touches. The touch system is based on frustrated total internal reflection (FTIR). Light sheets are coupled into a panel by emitters to propagate inside the panel by total internal reflection (TIR). When an object comes into contact with a touch surface of the panel, the propagating light is attenuated at the point of touch. The transmitted light is measured at a plurality of outcoupling points by one or more light detectors. The signals from the light detectors are processed for input into an image reconstruction algorithm that generates a 2D representation of interaction across the touch surface. This enables repeated determination of current position/size/-shape of touches in the 2D representation while one or more users interact with the touch surface.
To detect touches with high accuracy, the signal to noise ratio of the detected light shall be kept high. One source of noise is ambient light. Ambient light is light originating from sources of light outside the light guide others than the emitters. Ambient light can be removed from the detected light by creating an ambient light profile and subtracting the ambient light profile from the detected light. Another way or complement is to prevent the ambient light to reach the detectors, e.g. with a filter or shield. An example of an ambient filter is illustrated in WO2010056177A1, where a spectral bandpass filter is provided in front of the sensors. The filter is matched to transmit a major portion of the TIR radiation, and to block a major portion of the ambient light.
To provide a touch system with such a filter may be costly and careful placing of the components in relation to the filter is necessary to get the desired protection.
It is thus an object of the invention to provide an alternative out-coupling arrangement to a touch sensitive system that protects the at least one detector from ambient light, and that to at least some extent removes any noted deficiency with prior art.